This application relates generally to audio amplification systems and more specifically to methods and apparatus for avoiding or reducing feedback in audio amplification systems.
A typical audio amplification system, such as a public address system, acquires a desired audio input in the form of sound from one region of space (a stage, for example), then amplifies and projects it into another region (the auditorium, for example) in the form of audio output amplified sound. When a portion of the amplified audio output combines with the desired audio input at the input to the audio amplification system, a feedback loop is created. When the loop gain exceeds unity, i.e., the amplified sound received at the input is diminished from the output less than the amplification, objectionable ringing and oscillation (“howling”) can occur where the loudness of the sound output is only limited by the linearity and capacity of the audio amplification system. There have been a number of attempts to design audio amplification systems and sound spaces to deal with this runaway amplification, but they have limits and impose constraints.
One typical trigger that can create a positive feedback loop is acoustical region coupling. For this reason, traditional feedback avoidance strategy includes acoustical separation, directional transducers, and in-line system layout. In addition, appropriate use of absorptive and diffusive materials can minimize acoustical feedback paths. In practice, however, architectural-acoustic recommendations are often outweighed by many valid aesthetic, structural, operational, and economic preferences for large parallel expanses of sound reflectors.
Acoustical separation may not be an option, however, when staging that “breaks the fourth wall” is desirable. When microphones need to be made available in an audience area to enable individual audience or cast members to be heard by the entire audience, broadly overlapping sound acquisition and diffusion zones can trigger a positive feedback loop.
Suppression of feedback may also be accomplished by sending the audio input signal through a digital signal processor (“DSP”) prior to amplification. Some prior art audio systems that use DSPs rely on detection and suppression of feedback using after-the-fact gain control. This is often undesirable because there can still be a loud squeal between the time the runaway feedback begins and the time when the audio system detects and corrects for the runaway feedback.
Other prior art audio systems that use DSPs, for example in teleconferencing systems, might incorporate technologies such as echo cancellation and echo suppression, however, that is a slightly different problem and is somewhat simpler to deal with since there are two audio systems in the loop and the sound spaces are distinct. The problem is more difficult when the input sound space and the output sound space are not segregated and separated by any sound processing mechanisms but are free to have their sounds intermixed.